Stabilizing cellulose esters



Patented Feb. 25, 1941 UNITED STATES STABILIZING CELLULOSE ESTERS MervinE. Martin, Cumberland, Mal, assignor m CelaneseCorporation of America, acorporation of Delaware No Drawing. Application March 24, 1939, SerialNo. 263,909

20 Claims This invention relates to the preparation of organic esters ofcellulose, and relates more particularly to the production of stabilizedorganic esters of cellulose.

An object of this invention is the economic and expeditious preparationof stabilized organic esters of cellulose.

A further object of this invention is the provision of an improvedprocess for stabilizing organic esters of cellulose by the use ofphosphoric acid.

Other objects of this invention will appear from the following detaileddescription and claims.

In its broadest aspect the presentinvention is concerned with a processof stabilizing organic esters of cellulose by dissolving unstabilizedorganic esters of cellulose in a lower aliphatic acid such as, forexample, acetic acid, propionic acid and butyric acid, in the presenceof a small amount of phosphoric acid, and then reprecipitating theester.

In the process of preparing organic esters of cellulose, theesterification of cellulose is usually carried out by treating cellulosewith an organic acid anhydride in the presence of an organic aciddiluent or solvent for the ester of cellulose being formed and acatalyst such as sulphuric acid. After completion of the esterificationof the cellulose, there results a homogeneous viscous solution, andwater may then be added in amounts suflicient to convert any organicacid anhydride remaining into the corresponding acid, whereupon usuallyafter the addition of a certain quantity of water, the cellulose esteris permitted to hydrolyze or ripen to develop the desired solu-- bilitycharacteristics. Water and/or other nonsolvent for the cellulose estermay then be added in suflicient amounts to precipitate the celluloseester from solution. The cellulose ester is then washed and/or distilledto free it of acids as much as possible.

The present invention may be employed in the making of any suitableorganic ester of cellulose such as cellulose formate, cellulose acetate,cellulose propionate and cellulose. butyrate. Cellulose in any suitableform, such as cotton, cotton linters, wood pulp made by either thesulphite or soda process, reconstituted cellulose, etc. may be employedin making the cellulose esters. The cellulose may be activated bypretreatment with lower aliphatic acids or by treatment with alkali,etc. The esterifying agent may be formic acid, acetic anhydride,propionic anhydride or I butyric anhydride depending upon the ester tobe formed. The esterification may be carried out in the presence of asuitable catalyst such as sulphuric acid, sodium bisulphate, methylsulphate or a mixture of these with each other or with other catalystssuch as zinc chloride. However,.f0r the sake of simplicity thisinvention will be described in connection with the preparation. ofcellulose acetate which is at the present time commercially the mostimportant of the cellulose esters.

Thus, in preparing cellulose acetate by theprior process such asoutlined above for the preparation of cellulose esters, cellulose isacetylated by treatment with acetic anhydride and a catalyst, such assulphuric acid, in the presence of glacial acetic acid as a solvent forthe cellulose acetate that is formed. After completion of theacetylation, the resulting solution of cellulose acetate in glacialacetic acid is ripened and then treated with a large excess of water orother non-solvent to precipitate the cellulose acetate. Afterprecipitation the cellulose acetate may be washed with water to removeas much acid and other non-cellulose ester ingredients of theacetylation mixture as possible.

Prior to precipitation the cellulose acetate may be treated with basicsalts such as basic aluminum acetate, sodium aluminate, sodium acetateor with mixtures of these with aluminum chloride, zinc chloride orhydrochloric acid. This treatment acts to remove sulphur compounds andother non-stable compounds. It is, with a process wherein such basicsalts are used that this invention is particularly concerned.

It has been found that by treating the precipitated cellulose acetate,made in accordance with a process wherein basic salts have been employedto remove the sulphur and other non-stable compounds, by dissolving thecellulose acetate in a suitable solvent containing a small amount ofphosphoric acid, a cellulose acetate of greater stability and otherimproved physical properties is produced without any substantialalteration in the acetyl value or the acetone viscosity thereof.

In accordance with this invention, the cellulose acetate as it isprecipitated, washed neutral and dried is dissolved in a solventcomprising acetic acid of high concentration and containing a smallamount of phosphoric acid, the solution allowed to stand at a constanttemperature for a suitable length of time and the cellulose acetateprecipitated by the use of a large quantity of water or other suitablenon-solvent. If acetic acid below 96% concentration is employed as thesolvent, little or no improvement in the stability of the celluloseacetate is effected. The acetic acid should have a concentration of atleast 96% and preferably 99.5% for the purposes of this invention. Thephosphoric acid is conveniently added to the acetic acid in the form ofa solution of concentration. For example, cellulose acetate precipitatedfrom a solution in which a basic salt had been added to remove sulphuror other non-stable compounds may be stabilized by dissolving 10 partsof the same in 40 parts of acetic acid having a concentration of atleast 96% containing from 0.02 to 0.08 parts of phosphoric acid. The useof greater amounts of phosphoric acid causes the cellulose acetate tohydrolyze.

The product formed in accordance with this invention has substantiallythe same acetyl value and acetone viscosity of the cellulose acetatebefore treatment but its stability and heat tests are much greater.Thus, cellulose acetate prepared by the process of this invention ismore stable to light and heat and has a higher heat test than thecellulose acetate before treatment. That is, it may be heated to ahigher temperature before it begins to decompose or char.

In order to further illustrate this invention but without being limitedthereto, the following specific example is given:

Example Cellulose acetate is formed by treating cellulose acid. Afterthis mixture is allowed to stand for 24 hours at 22 C. the celluloseacetate is precipitated by the addition of a large quantity of water andthen washed and dried. The cellulose acetate thus purified has aheattest of 25 degrees higher than the cellulose acetate, which had not beensubjected to the treatment in accordance with this invention but whichwas merely washed and dried after precipitation from the solution ofwhich it was formed, is more stable to light and heat and has anextremely low sulphur content. The acetone viscosity and the acetylvalue of the cellulose acetate before and after treatment aresubstantially the same.

.Cellulose acetate produced in accordance with this invention iseminently suitable for the making of yarns, filaments, staple fibers,bristles, straw, foils, films, plastics, molding compositions, coatingcompositions and other uses to which cellulose acetate is put.

It is to be understood that the foregoing de-v tailed description isgiven merely by way of illustration andthat many variations may be madetherein without departing from the spirit of my invention.

Having described my invention, what I desire to secure by Letters Patentis:

1. Process for stabilizing a lower fatty acid ester of cellulose, whichcomprises dissolving an unstabilized organic acid ester of cellulose ina solvent therefor of a concentration of at least 96% containingphosphoric acid in a proportion of 0.2 to 0.8% based on the Weight ofthe cellulose ester, and then precipitating the ester from the solution.

2. Process for stabilizing cellulose acetate,

which comprises dissolving the unstabilized cellulose acetate in asolvent therefor of a concentration of at least 96% containingphosphoric acid in a proportion of 0.2 to 0.8% based on the weight ofthe cellulose acetate, and then precipitating the cellulose acetate fromthe solution.

3. Process for stabilizing a lower fatty acid ester of cellulose, whichcomprises dissolving an unstabilized organic acid ester of cellulose ina lower aliphatic acid of a concentration of at least 96% containingphosphoric acid in a proportion of 0.2 to 0.8% based on the weight ofthe cellulose ester, and then precipitating the ester from the solution.

4. Process for stabilizing cellulose acetate, which comprises dissolvingthe unstabilized cellulose acetate in acetic acid of a concentration ofat least 96% containing phosphoric acid in a proportion of 0.2 to 0.8%based on the weight of the cellulose acetate, and then precipitating thecellulose acetate from the solution.

5. Process for stabilizing cellulose acetate, which comprises dissolvingthe unstabilized cellulose acetate in acetic acid of a concentration ofat least 99.5% containing 85% phosphoric acid in a proportion of 0.2 to0.8% based on the weight of the cellulose acetate, and thenprecipitating the cellulose acetate from the solution.

6. Process for stabilizing a lower fatty acid ester of cellulose thathas been precipitated in the presence of a basic salt, which comprises'dissolving the organic acid ester of cellulose in a solvent therefor ofa concentration of at least 96 containing phosphoric acid in aproportion of 0.2 to 0.8% based on the Weight of the cellulose ester,and then precipitating the ester from the solution.

'7. Process for stabilizing cellulose acetate that has been precipitatedin the presence of a basic salt, which comprises dissolvingthe celluloseacetate in a solvent therefor of a concentration of at least 96%containing phosphoric acid in a proportion of 0.2 to 0.8% based on theweight of the cellulose acetate, and then precipitating the cellu-- loseacetate from the solution.

8. Process for stabilizing a lower fatty acid ester of cellulose thathas been precipitated in the presence of a basic salt, which comprisesdissolving the organic acid ester of cellulose in a lower aliphatic acidof a concentration of at least 96% containing phosphoric acid in aproportion of 0.2 to 0.8% based on the weight of the cellulose ester,and then precipitating the ester from the solution.

9. Process for stabilizing cellulose acetate that has been precipitatedin the presence of a basic salt, which comprises dissolving thecellulose acetate in acetic acid of a concentration of at least 96%containing phosphoric acid in a proportion of 0.2 to 0.8% based on theweight of the cellulose acetate, and then precipitating the celluloseacetate from the solution.

10. Process for stabilizing cellulose acetate that has been precipitatedin the presence of a basic salt, which comprises dissolving thecellulose acetate in acetic acid of a concentration of at least 99.5%containing 85% phosphoric acid in a proportion of 0.2 to 0.8% based onthe weight of the cellulose acetate, and then precipitating thecellulose acetate from the solution.

11. Process for stabilizing cellulose acetate that has been precipitatedin the presence of a basic salt, which comprises dissolving thecellulose acetate in acetic acid of a concentration of at least 96%containing phosphoric acid in a proacetate in 40 parts of acetic acid ofa concentraportion of 0.2 to 0.8% based on the weight of the celluloseacetate, permitting the solution to stand for 24 hours at a lowtemperature, and then precipitating the cellulose acetate from thesolution.

13. Process for stabilizing cellulose acetate that has been precipitatedin the presence of a basic salt, which comprises dissolving thecellulose acetate in acetic acid of a concentration of at least 96%containing phosphoric acid in a proportion of 0.2 to 0.8% based. on theweight of the cellulose acetate, permitting the solution to stand for2'4 hours at a temperature of 22 C., and then precipitating thecellulose acetate from the solution.

14. Process for stabilizing cellulose acetate that has been precipitatedin the presence of a basic salt, which comprises dissolving thecellulose acetate in acetic acid of a concentration of at least 99.5%containing 85% phosphoric acid in a proportion of 0.2 to 0.8% based onthe weight of the cellulose acetate, permitting the solution to standfor 24 hours at a temperature of 22 C., and then precipitating thecellulose acetate from the solution.

15. Process for stabilizing cellulose acetate, which comprisesdissolving 10 parts of cellulose acetate in 40 parts of acetic acid of aconcentration of 99.5% containing from 0.02 to 0.08 part of phosphoricacid, permitting the solution to stand for 24 hours at a temperature of22 C., and then precipitating the cellulose acetate from the solugotion.

16. Process for stabilizing cellulose acetate, which comprisesdissolving 10 parts of cellulose tion of 99.5% containing substantially0.08 part of 85% phosphoric acid, permitting the solution to stand for24 hours at a temperature of 22 C. and then precipitating the celluloseacetate from 5 the solution.

17. Process for stabilizing cellulose acetate that has been precipitatedin the presence of a basic salt, which comprises dissolving parts ofcellu. lose acetate in 40 parts of acetic acid of a concentration of99.5% containing from 0.02 to 0.08 part of phosphoric acid, permittingthe solution to stand for 24 hours at a temperature of 22 C., and thenprecipitating the cellulose acetate from the solution.

13. Process for stabilizing cellulose acetate that has been precipitatedin the presence of a basic salt, which comprises dissolving 10 parts ofcellulose acetate in 40 parts of acetic acid of a concentration of 99.5%containing 0.08part of 85% phosphoric acid, permitting the solution tostand I for 24 hours at a temperature of 22 C., and then precipitatingthe cellulose acetate from the solution.

ester of cellulose, which comprises dissolving a ripened andunstabilized organic acid ester of cellulose in a solvent therefor of aconcentration of at least 96% containing phosphoric acid in an amount of0.2 to 0.8% based on the weight of the cellulose ester, and thenprecipitating the ester from the solution; 1 I

20. Process for stabilizing a lower fatty acid ester of cellulose, whichcomprises dissolving a ripened and unstabilized organic acid ester ofcellulose in a lower aliphatic acid of a concentration of at least 96%containing phosphoric acid in an amount of 0.2 to 0.8% based on theweight of the cellulose ester, and then precipitating the ester from thesolution.

MERVIN E. MARTIN.

19. Process for stabilizing a lower fatty acid 2 5

